The behavior of reorientational correlation functions of water at the water-lipid bilayer interface.

We studied the effects of confinement and the head group motion on the behavior of the reorientational correlation functions for water molecules at the water/lipid bilayer interface. The correlation functions were calculated from the data obtained from two molecular dynamics simulations: one with a flexible bilayer and the other with a frozen bilayer. In our present analysis the water molecules were separated into spatial regions according to their distance from the bilayer surface and into population groups, according to the length of their stay in the corresponding regions. We estimate that for most of the water molecules that are in a strongly confined environment of the transition region between the head groups and tails, and that solvate carbonyl groups, the decay time of their reorientational correlation functions is of the order of a few tens of picoseconds. Water molecules that stay inside the transition region for long periods of time can display longer time decay (of the order of hundreds of picoseconds). This latter long time decay is determined by the dynamics of the phospholipids, it is substantially reduced when the bilayer is frozen. The decay of the correlation functions for the interfacial water molecules that are solvating the head groups is also slowed down when compared to bulk, but just by factors of 3-4.